The present invention relates to providing an input/output selection switch of a radio communication device and in particular, a fast charge means for a capacitor in a negative bias generation circuit.
A radio transceiver is generally used widely. For a mobile telephone etc., the UMTS (W-CDMA) system using CDMA (Code Division Multiple Access) is the mainstream of the multiple access system, however, a radio transceiver that uses TDMA (Time Division Multiple Access) is still used widely.
In a radio transceiver that employs the TDMA system, it is common to use one antenna by switching between transmission and reception. FIG. 1 is a conceptual diagram of an antenna switch circuit 1000 that switches between the transmission side and the reception side of the antenna.
The antenna switch circuit 1000 has two terminals, that is, a transmission input terminal and a reception output terminal. The antenna switch circuit is a circuit for the purpose of electrically coupling the transmission input terminal and the antenna for transmission and the reception output terminal and the antenna for reception, respectively.
The antenna switch circuit 1000 includes antenna switch transistors MN1, MN2, MN3 and MN4. As the antenna switch transistors MN1, MN2, MN3 and MN4, conventionally, a pHEMT (gallium arsenide) FET is used generally.
The antenna switch transistors MN1 and MN4 operate as a switch for grounding and the antenna switch transistors MN2 and MN3 operate as a switch for antenna connection, respectively. Consequently, when the transmission input terminal is connected to the antenna, the antenna switch transistor MN2 turns ON and on the other hand, the antenna switch transistor MN1 turns OFF. This is the same for transmission, and thus, the operation of the antenna switch transistor MN1 is always opposite to that of the antenna switch transistor MN2. This is the same for the antenna switch transistor MN3 and the antenna switch transistor MN4.
The source and drain of each of these antenna switch transistors are coupled by a resistor having a high resistance value. Consequently, the potential of the source is the same as that of the drain and further, the antenna switch transistors MN1 and MN4 are grounded, and thus, all of the potentials of source/drain terminals of these transistors are the GND potential.
When a switching control circuit 1001 controls the potential of a gate terminal of each of these antenna switch transistors, the turning ON/OFF of these antenna switch transistors can be controlled. When a negative bias is not used, a control output of the switching control circuit 1001 is either the GND potential or the VDD potential.
In transmission, a signal having an amplitude Vpp that fluctuates positively and negatively passes through the transmission input terminal. In order for the antenna switch transistor to turn OFF, the source/drain potential needs to be greater than or equal to the potential of a gate terminal. There may be however a case where it becomes less than or equal to the gate potential at the time of the lower limit peak of a signal to be input to the transmission input terminal depending on the amplitude amount of the signal to be input to the transmission input terminal.
To the drain of the antenna switch transistor MN1, a potential of a several volts is applied usually. Consequently, unless the source/drain potential is greater than or equal to the gate potential, (A) point in the figure is short-circuited as a result. This short circuit causes the waveform at the (A) point on the negative side to be clipped at GND, resulting in a distortion of the waveform.
FIG. 2 is a circuit diagram showing a configuration of a conventional pHEMT SW.
The conventional pHEMT SW is configured to include a step-up circuit 2001 and a SW transistor 2002.
The step-up circuit 2001 steps up a control voltage and gives a higher potential to the gate voltage than to the voltage between the source and drain of the SW transistor 2002. Due to this, the same effect as that of the negative bias is obtained. Software is designed to be capable of operating at high speed so that its start-up time may respond to the charge time of the step-up circuit.
For such a problem that the waveform is clipped at GND, U.S. Pat. No. 6,804,502 (Patent Document 1) discloses a technique to prevent the waveform from being clipped at GND by causing the control circuit to generate a negative bias.